[anttiok]

For one, the microkernel would be missing ;)

More seriously though, you're absolutely right in that it's a step in the direction of an "optional" microkernel architecture. That's actually how rump kernels on NetBSD started: running the kernel file system driver as a library in userspace on top of a FUSE-like subsystem. It's pretty useful functionality, since it allows you to handle untrusted file system images safely in userspace, while not imposing performance penalty on the trusted images which can be handled by the same driver running in the kernel. Unlike with FUSE-specific drivers, you don't run into issues with unsymmetric driver support in userspace vs. kernel.